To prevent fuel or fuel vapors from fuel tanks of motor vehicles from escaping into the environment, it is mandatory in many states that the tightness of these fuel tanks be monitored. For this purpose a pressure switch is often used whose switching status indicates that a given switching pressure in the fuel tank itself or in the fuel tank ventilation means is exceeded or not reached.
For example, U.S. Pat. No. 5,263,462 discloses a method in which by using such a pressure switch and a temperature sensor it is ascertained whether as a result of cooling of the tank ventilation means after shutting off of the internal combustion engine within the tank ventilation means a natural underpressure forms which leads to closing of the pressure switch.
Furthermore, DE 10 2006 045 678 A1 discloses a device of the initially mentioned type in which the tank ventilation means in addition to the pressure switch comprises a fuel vapor storage tank which communicates with the interior of the fuel tank and a controllable tank ventilation valve which is located between the storage tank and the intake manifold of the internal combustion engine, and a method of the initially named type in which the serviceability of the tank ventilation means is assessed based on a comparison of the switching status of the pressure switch before and after opening of the tank ventilation valve.
In all methods and devices in which a pressure switch is used for ascertaining leaks of a fuel tank, however, proper operation of the pressure switch must always be guaranteed since in a defect or malfunction of the pressure switch reliable monitoring of the tightness of the fuel tank is no longer guaranteed. For this reason it is even required by law in some states that proper operation of the pressure switch be tested or monitored.
In known methods and devices for testing the tightness of a fuel tank of an internal combustion engine, the proper operation of the pressure switch is usually tested in a so-called coasting phase of an engine control device of the internal combustion engine in which the crankshaft of the internal combustion engine is no longer turning, the electronics and, in particular, the engine control device, however, are still fully serviceable. As is described, for example, in DE 10 2006 045 678 A1, advantageously for this test the controllable tank ventilation valve in the connecting line between the storage tank for the fuel vapors and the intake manifold of the internal combustion engine is open, the switching status of the pressure switch being detected before and after opening of the tank ventilation valve. When the pressure switch is closed before opening of the tank ventilation valve as a result of a negative pressure in the fuel tank, and opens when the tank ventilation valve opens as a result of the pressure rise during venting, it is diagnosed as free of faults. When the pressure switch conversely does not open, it is diagnosed as faulty.
This known method, however, has two major disadvantages. On the one hand, it can be carried out only during operation of the internal combustion engine and during the coasting phase of the engine control device, but not while the internal combustion engine or a vehicle equipped with the internal combustion engine, in particular a motor vehicle, is shut off. On the other hand, the method is not suited for internal combustion engines with exhaust gas turbochargers, since in the connecting line which discharges upstream from the compressor of the exhaust gas turbocharger into the intake manifold and which leads from the storage tank for the fuel vapors to the intake manifold, in addition to the tank ventilation valve a check valve is also installed which prevents backflow into the fuel tank and thus its venting by way of the tank ventilation valve.
On this basis the object of the invention is to improve a method and a device of the initially mentioned type such that even with the internal combustion engine shut off or the vehicle shut down and for internal combustion engines with an exhaust gas turbocharger, diagnosis of the pressure switch is possible.